a. Field of the invention.
The present invention pertains to a rear view mirror apparatus for automobiles. More specifically, the present invention relates to a rear view mirror apparatus of periscope type for automobiles utilizing three planar mirrors and arranged to obtain rear views.
B. Description of the prior art
The typical structure of known rear view apparatuses is as shown in FIG. 7. More specifically, such a structure features an arrangement having a reflecting mirror 102 for reflecting the light rays reflected from an objective mirror 101. Said reflecting mirror is mounted on the rearward edge of an opening 110 formed through the top or roof 100 of the driver's chamber of an automobile and relects said light rays forwardly and downwardly therefrom. The latter reflected light rays are again reflected to an eye-piece mirror 103 having a structure similar to that of an ordinary room mirror and directed toward the eyes of the driver. Such a known rear view mirror of periscope type, however, has the following problems.
1. The housing 105 for containing the objective mirror 101 must be of a substantial height H and protrude from the top of the driver's chamber. This is not only undesirable from the viewpoint of design, but also will cause increased resistance to the automobile itself during running, for the following reasons.
The objective mirror 101 must be positioned so that its lowermost end 101a will be located above of the uppermost end 102a of the reflecting mirror 102 so that the objective mirror 101 may catch the light rays coming from the rear. Accordingly, the distance H which the housing protrudes from the roof 100 of the automobile will be: H .gtoreq. h.sub.1 + h.sub.2 (wherein: h.sub.1 represents the height of the objective mirror 101; h.sub.2 represents the distance with which the reflecting mirror 102 projects above the top 100 of the automobile). It should be noted, however, that into the reflecting mirror 102 protrudes to an excessive extent in the driver's chamber, the latter space will become smaller accordingly, thereby causing aggravation to the driving conditions as well as obstructing the forward view, and, furthermore, will be hazardous to the driver should he hit his forehead against the protruding reflecting mirror. Therefore, the reflecting mirror must to be positioned as high as possible in the driver's chamber. This, in turn, will cause the distance h.sub.2 to increase as a matter of course. On the other hand, since the position of the reflecting mirror 102 is predetermined as stated above, it is not possible to have the angle of inclination .theta. of the objective mirror 101 smaller than a certain value. Accordingly, the magnitude of h.sub.1 also cannot be made smaller than a certain level. As a result, the value of H will be substantially great as a matter of course.
2. As shown in FIGS. 6 and 7, the movement on b' and c' of the range of field of vision on the faces of the reflecting mirror 102 and the objective mirror 101 relative to the amount of movement of the eye, will be great. Accordingly, there must be provided a larger objective mirror 101 and reflecting mirror 102 to encompass the total range of the field of vision. Thus, the apparatus itself will have an undesirably large size. This is quite disadvantageous from the viewpoints of the operational efficiency of mounting the apparatus onto the vehicle, the design and the cost.
Such disadvantages are caused by the fact that the optical distance (the length of the path of the light rays) between the respective mirror surfaces is excessively great when compared with the length of the path of light rays from the eye to the eye-piece mirror 103. Let us now assume that the length of the path of light rays from the eye to the eye-piece mirror 103 is designated as l.sub.1, that the length of the path of light rays from the eyepiece mirror 103 to the reflecting mirror 102 is designated as l'.sub.2, and that the length of the path of light rays from the reflecting mirror 102 to the objective mirror 101 is designated as l'.sub.3. Now, the foregoing disadvantages can be explained by using the aforesaid designations. There can be set forth the following relationship between the amount of movement of the eye and the movement b' and c'of the range of the field of vision at the respective mirror surfaces of the reflecting mirror 102 and the objective mirror 101: ##EQU1##
3. Also, since the reflecting mirror 102 is large as stated above, it is difficult to clean the surfaces of the objective mirror 101 and the window 106.
In order to eliminate the aforesaid drawbacks and inconveniences, there is proposed a rear view apparatus of periscope type designed to have a compact size by the combined use of a convex lens and a concave mirror. However, such an optical system will develop changes in the magnification of the image, a distorted image or disappearance of the image as the position of the eye is altered.
Furthermore, the convex lens and a concave mirror apparatus requires processing with a very high degree of precision in order to avoid the distortion of the image reflected thereby, and also requires a very strict postioning of components and a precision arrangment. Thus, this known apparatus will undesirably be accompanied by a high cost of parts along with a decrease in productivity. As such, such an apparatus will require a cost which may be several times or several tens of times more than that of the apparatus utilizing planar mirrors.